Flawless sperm functioning is essential for the successful completion of fertilization in humans. This functioning includes activities of the sperm prior to its meeting the oocyte, sperm-oocyte plasma membrane fusion, and the cytoplasmic events mediated by the sperm nucleus and centrosome that conclude the fertilization process the merging of the parental genomes in the activated zygote. Defects in sperm behavior at any stage result in the inability to complete fertilization, and quantitative analyses of these defects have been developed into assays for male infertility. These assays determine the extent of aberrations in sperm number, morphology, motility, the ability to undergo the acrosome reaction, the release of acrosome enzymes, and penetration assays into zona-free hamster oocytes or isolated zonae. This application investigating idiopathic male infertility studies the microtubule organizing capacity of the human sperm centrosome. To investigate the extent to which defects in the human sperm centrosome are causes of male infertility, questions are posed in four specific aims using sperm from fertile and infertile men. Aim #1 Is centrin, a centrosomal protein, found in human sperm, and does its concentration vary predictably in sperm donated from men of differing fertility? Aim #2. Does the binding of maternal centrosomal proteins, especially -tubulin, vary predictably in sperm from men of differing fertility? Aim #3. Are there differences in the ability of sperm from men with varying fertility to nucleate and direct the assembly of microtubule-containing asters in extracts from Xenopus eggs? Aim #4. Will the sperm asters formed in pronucleate-stage mammalian oocytes, inseminated with sperm from men of varying fertility, predictably vary in their size and microtubule density?